The functions of temperature regulation (closed-loop control) aging compensation of glow plugs as well as malfunction prediction of glow plugs require knowledge of when the glow plug was exchanged. The obvious possibility of indicating an exchange of glow plugs to the control device is available via a standard diagnosis tester. Such a device is used by a service station employee to notify the system via a diagnosis interface whether and, if so, which glow plug was exchanged. However, some customers do not accept this approach because it constitutes an additional error source, which offers a questionable fallback level or a questionable safety concept. More specifically, in case of an incorrect entry, it is unclear what will happen to the functions that rely on this variable.